A first aspect of the present invention relates to designing prostheses in general and more particularly femoral head prostheses and acetabulum prostheses.
Prior art in this field comprise:
Article from Biomedizinische Technik, Vol. 30, Nr. 5, G. Giebel et al.: "Fertigung von Knochenmodellen nach Computer-Tomographie-Daten zur Verwendung in Chirurgie und Orthopadie" (pages 111-114);
Lecture during First European Congress of Knee Surgery and Arthroscopy, Apr. 9-14 1984, Berlin, issued by E. L. Trickey et al. Springer-verlag, Berlin, Heidelberg, DE U. Munzinger et al.: "Biomechanics of Knee Prostheses" (pages 324-334);
U.S. Pat. No. 4,153,953 (Grobbelaar), column 3, pages 19-28, 56-61;
DE-A-2805868 (Engelbrecht et al.), FIG. 1;
GB-A-2096003 (Burstein), page 2;
EP-A-0149527 (Lee et al.), FIGS. 1, 2
GB-A-2045082 (Raab), page 1
It is a first object of the method according to the present invention to improve upon the prior art.
It is a further object to provide a method in which tensile, compressive and shear stresses acting on the interface between prosthesis and bone, whether or not provided with a cement mantel, are controllable. Theories exist that resorption or necrosis of bone are caused by stress shielding of the bone due to the prosthesis.
A preferred method according to the present invention concerns prostheses minimizing local normal and shear stresses between bone and prosthesis. Existing endoprostheses used in practice generally consist of a metal core surrounded by a mantle of an artificial material which is in direct contact with the bone. According to the present invention, a mathematical simulation model is made of such a construction, by means of the so-called finite element method. With this model the stresses which act on the connecting planes or at an interface area between the artificial mantle and the bone can be calculated. These stresses depend, amongst others, on the geometry of the prosthesis. By means of a mathematical search procedure that geometry can be determined which creates a prechosen stress pattern at the interface between bone and prosthesis.
The above-mentioned method was applied to a 2-D model of a femoral head prosthesis and a acetabular prosthesis.
A second aspect of the present invention relates to a femoral head prosthesis having a specific shape.
The characteristics of this shape are not known from the prior art, e.g.:
DE-A-3.243.861; DE-A-2.805.868; BE-A-2.247.721; GB-A-2.078.523; GB-A+2.045.082; EP-A-146.192; EP-A-12.146; and U.S. Pat. No. 4,021,865.
A third aspect of the present invention is directed to a method for the positioning of an endo-prosthesis in a bone as well as spacing or positioning elements.
From the prior art a more cumbersome method for introducing a femoral head prosthesis is known from U.S. Pat. No. 4,718,909.
According to the fourth aspect of the present invention an acetabular prosthesis having a characteristic shape is provided. Those characteristic shapes are not known from the prior art, viz. U.S. Pat. Nos. 4,566,138 and 4,563,778.
Malfunctioning of a human joint due to arthrosis, arthritis, trauma conditions or any other cause reduces the well-being of the person concerned and is expensive for society at large. With increasing success, such defects are corrected by joint arthroplasty. Annually in the world, these procedures are carried out more than 500,000 times. Most defects concern the hip and the knee joints; however, ankles, toes, shoulders, elbows, wrists and finger joints can also be provided with artificial joints.
The known prostheses usually consist of a combination of a metal and a plastic. Most prostheses have a metal stem or plateau which is fixed within the bone by means of acrylic cement. Acrylic cement is a fast curing mixture of monomethyl-methacrylate liquid and polymethylmethacrylate powder. This viscous mass can be introduced into the intramedullary canal or into the trabecular bone of a bone. Subsequently, the prosthesis is inserted in the acrylic cement mass. After curing, a mantle of cement remains in between the metal core and the bone. The bone cement does not have any adhesive properties, and the connection between the cement and the bone is merely realized by mechanical interlock. A firm interlock is achieved by a sufficiently deep penetration of the cement into the trabecular bone. This penetration arises if the acrylic cement has a sufficiently low viscosity and is pressurized for some time.
In cases where no acrylic cement is used, the prosthesis is introduced directly into the bone. Such a prosthesis can be made out of metal completely, or can be composed out of a metal core and a coating. In this case there is not primary fixation between the prosthesis and the bone. Such a fixation can, however, be accomplished by the ingrowth of bone into the surface structures of the prosthesis.
A well-known long-term complication of surgical joint replacement is loosening of the prosthesis. This loosening mostly occurs at the interface between prosthesis and bone. It is known that the chances for this complication to occur increase with decreasing age of the patient at the time of the operation and an accordingly increased activity level. Loosening of the prosthesis leads to resorption of bone and finally to severe complaints of pain. These complaints can be so dramatic that a revision operation is indicated, in which the old prosthesis is removed and a new one is inserted. However, the long-term results of a revision operation are not as good as those of a primary joint replacement, and the revision procedure cannot be repeated an arbitrary number of times.
No common opinion exists presently on the cause or etiology of loosening. The following possibilities have been suggested: necrosis of the bone at the interface caused by toxicity of the cement monomer or excessive heat release due to the exothermally curing bone cement, fracture of the cement mantle due to mechanical loading, failure of the interface caused by local stress peaks, reduced loading of the bone caused by stress shielding, etc.
Further advantages, features and details of the invention will be clarified by reference to the accompanying drawings.